How to rate flashlights?

I'm looking for a "tactical" flashlight to hang under my shotgun. But I've noticed that some are rated in watts, 1w, 2w, 3w, etc. and others are rated in lumens. Is there anyway to compare the two? How many lumens does it take to temporarily dis-orient a bad guy? Some web sites say over 100 will do it. What have you heard?

ive many differing opinions regarding how many lumens it will take to disorient a bad guy. i wouldnt get a flashlight for THAT purpose, sure itll be bright but it would be more useful illuminating whatever is in front of you. if you point it at homeboy the doorkicker, be prepared to shoot regardlessly.

dont hang a flashlight off of a shotgun. most of those clamps and mounts suck. not to mention a lot of flashlights out there are cheaply made.

surefire has released LED versions of their flashlights mounted in shotgun slides (pumps). i recommend one of those if this weapon will be keeping you safe from danger

I'm looking for a "tactical" flashlight to hang under my shotgun. But I've noticed that some are rated in watts, 1w, 2w, 3w, etc. and others are rated in lumens. Is there anyway to compare the two? How many lumens does it take to temporarily dis-orient a bad guy? Some web sites say over 100 will do it. What have you heard?

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There are three common ratings on flashlight output - power dissipation (watts), beam intensity (candlepower), and total output (lumens).

Using watts to describe the output of a light source is meaningless. It is mostly used for LED flashlights. The 5W LED of four years ago is nearly being edged out by the screaming-bright 1W LED that just hit store shelves last week. The technology is changing that fast. All that the watt stat usually tells you is what the maximum rated power is for the LED. Odds are it's not being driven at said power and even if it is, ambient temperature and the thermal characteristics of the flashlight make an enormous difference in output. Do not buy flashlights based on wattage.

Candlepower is more useful - it describes the measured brightness of the beam in candella, the base unit for lumens. The problems with beam candlepower is that it doesn't describe overall output, it usually describes peak beam candlepower, and is generally exaggerated greatly. A relatively small mount of light can be concentrated into a surprisingly high candlepower number with the right reflector or optics setup and creative measurement standards. For a while Inova had a small LED flashlight that produced a ~5 degree beam with zero spill; its candlepower numbers were great but it was almost useless because at waist height it illuminated an area ~10" in diameter and almost nothing else. Peak beam candlepower ratings are to be taken with a grain of salt (or in the case of spotlights, a trainload of salt) because they are nearly always gloriously inflated; "1 million candlepower" spotlights rarely meter 200,000 candlepower in the real world.

Lux are sometimes seen - it's just the metric equivalent of candlepower. I think 1 lux at one meter is the same as 1 candlepower at one foot.

Lumens are probably more useful than candlepower because they describe overall output. While a focused 10 lumen light source can be made brighter than a diffused 100 lumen light source, all other things being equal between two competing products the one with more lumens will be brighter. Much like beam candlepower, lumens tend to be exaggerated - or in the case of many LED flashlights, simply the best-case output for the LED under lab conditions rather than measured output from a completed flashlight.

Just bear in mind that human perception of light output is not linear. Doubling the output (or brightness) on the meter is perceived as being a ~50% increase in output. It is around a 4x increase in measured brightness that the mind perceives twice the light. If you can't decide between a 60 lumen light you like slightly better than an 80 lumen light, all other things being equal get the 60 lumen model.

Ideally, manufacturers would give you some idea of beam shape, beam intensity, lumens, and maybe the maximum distance than it would produce some minimum brightness (say, 1 candlepower) and the size of that spot. But that would be clinical standardization that would bring engineering into the equation rather than the exaggerated claims some marketing huckster dreams up.

Also - don't buy into that crap about being able to momentary blind or stun someone with a flashlight. There are too many factors at work there including dark adaptation, low level physiological reactions to sudden changes in light levels, and individual reactions / level of discipline.